Abstract
Ultra-thin CdTe:As/Cd1-xZnxS photovoltaic solar cells with an absorber thickness of 0.5 μm were deposited by metal-organic chemical vapour deposition on indium tin oxide coated boro-aluminosilicate substrates. The Zn precursor concentration was varied to compensate for Zn leaching effects after CdCl2 activation treatment. Analysis of the solar cell composition and structure by X-ray photoelectron spectroscopy depth profiling and X-ray diffraction showed that higher concentrations of Zn in the Cd1-xZnxS window layer resulted in suppression of S diffusion across the CdTe/Cd1-xZnxS interface after CdCl2 activation treatment. Excessive Zn content in the Cd1-xZnxS alloy preserved the spectral response in the blue region of the solar spectrum, but increased series resistance for the solar cells. A modest increase in the Zn content of the Cd1-xZnxS alloy together with a post-deposition air anneal resulted in an improved blue response and an enhanced open circuit voltage and fill factor. This device yielded a mean efficiency of 8.3% over 8 cells (0.25 cm2 cell area) and best cell efficiency of 8.8%.
Highlights
Thin film cadmium telluride (CdTe) has been an established technology for photovoltaic (PV) solar energy for a number of years [1]
The interface regions are not clearly defined due to individual layer roughness and interdiffusion processes resulting from the activation treatment
The progressive increase in Zn content in the window layer results in a corresponding depletion of Cd across the CdTe/Cd1-xZnxS interface as the Zn replaces the Cd, with the gradient across the junction becoming more prominent as the Zn concentration increases in the Cd1-xZnxS layer
Summary
Thin film cadmium telluride (CdTe) has been an established technology for photovoltaic (PV) solar energy for a number of years [1]. The rate at which Te can be supplied is largely dependent on the mining of Cu and its availability may become limited if the demand of global Te increased significantly [4]. Reduction of CdTe absorber thickness becomes an important consideration for continued large scale production of CdTe solar modules [6,7,8]. The use of less material will have a positive impact for reducing manufacturing costs and carbon footprint providing that the PV conversion efficiency does not deteriorate significantly. Ultrathin (≤ 1 μm) CdTe solar cells are more susceptible to lateral inhomogeneity across the device [9, 10] which is a limiting factor for industrial production
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